Method and system for multiple tuner application using a low noise broadband distribution amplifier

ABSTRACT

An active splitter circuit arrangement includes a first amplification module having a number of first input ports and first output ports. The first amplification module is configured to provide first stage amplification to a received input signal and produce from the amplified input signal a number of output signals, each substantially matching the input signal. Also included is a first gain control device having a number of gain input ports respectively coupled to the first output ports and a gain output port coupled to at least one of the first input ports. The first gain control device is configured to control a gain of the first amplification module. Next, a number of second amplification modules corresponding to the number of output signals has a number of second input ports respectively coupled to the first output ports. Each second amplification module is configured to receive a control signal from the second gain control device, provide second stage amplification to a corresponding one of the number of output signals based upon the control signal and produce an amplified output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/312,086 filed Aug. 15, 2001, and U.S. Provisional Application No.60/363,545, filed Mar. 13, 2002, which are both incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of low noiseamplifiers (LNAs) used in tuners. More specifically, the presentinvention relates to LNAs used in the tuners of set-top boxes, cablemodems, and high definition television (HDTV) devices. Tuners of thistype can also be used in personal video recorders (PVRs), out-of-band(OOB) devices, analog TV, and voice over internet protocol (VOIP)devices, as well as numerous other applications.

2. Related Art

Due to the complexity and availability of entertainment program sourcematerial, multiple television tuners are included in most set-top cableboxes and cable modems. These additional features, material, andfunctionality cause the set-top boxes to be significantly more complexand include increased functionality. Examples of this increasedfunctionality include the ability to watch simultaneous channels, suchas the popular picture-in-picture (PIP) television viewing mode and/orthe ability to watch one channel while simultaneously recording anotherchannel using a video cassette or a personal video recorder (PVR). Intraditional cable-set boxes, passive splitters with sufficient bandwidthare used to split the incoming cable signal into multiple outputs inorder to drive multiple tuners to provide PIP or facilitate the viewingof one channel and the simultaneous recording of another. Unfortunately,however, the use of the passive splitters degrades the overallperformance of the tuner.

For example, passive splitters are known to create insertion losses ofabout 6 dB, which significantly degrades the performance of theassociated tuners. This level of degradation, in turn, impacts theoverall system's noise figure and signal-to-noise ratio (SNR). Anadditional drawback of passive splitters in cable set-top boxes is thatthey do not permit the full use of cable signal ranges. Finally, passivesplitters require extensive printed circuit board area and necessitatebulky metal configurations for electromagnetic shielding in order tominimize interference.

What is needed, therefore, is an alternative to the passive splitterscurrently used in cable television tuners or cable modem typeapplications. More specifically, what is needed is a tunerimplementation that can improve upon the performance of passivesplitters, such that splitting the input signal into multiple signalswill not significantly degrade the overall performance of the tuner.This improved approach must facilitate the full use of the associatedcable signal ranges. The improved approach must also reduce spacerequirements, reduce cost, and ease many of the manufacturing issuessurrounding production of conventional tuners used in the aforementionedapplications.

What is needed is a technique to provide autonomous automatic gaincontrol (AGC) within the amplification modules associated with cabletuning and similar systems. Additionally, what is needed is an improvedtuner with autonomous AGC that can be integrated onto a singleintegrated circuit (IC) and be implemented, for example, incomplimentary metal-oxide semiconductor (CMOS) technology.

SUMMARY OF THE INVENTION

Consistent with the principles of the present invention as embodied andbroadly described herein, an exemplary tuner system includes a signalsplitting mechanism configured to receive and amplify a first inputsignal. The signal splitting mechanism also is configured to producefrom the amplified first signal a number of second signals, each of thesecond signals being substantially identical to the amplified firstsignal. Finally, the signal splitting mechanism is configured to outputeach of the amplified second signals. Next, the exemplary systemincludes a number of tuners corresponding to the number of secondsignals. Each tuner includes a signal amplification function and isconfigured to receive one of the second signals output from the signalsplitting mechanism and derive one or more informational signalstherefrom.

The present invention also includes an exemplary method for processing asignal in an amplifier including a first amplification module, a numberof second amplification modules, and a gain control device. The methodcomprises receiving a transmitted signal in the first amplificationmodule and applying a first level of amplification thereto. Next, themethod includes providing the first level amplified signal to each ofthe number of second amplification modules to produce a number of secondlevel amplified signals. Each of the number of second level amplifiedsignals has characteristics substantially similar to the other of thenumber of second level amplified signals. The method also includesdetecting a signal level of each of the number of second level amplifiedsignals and producing a gain control signal based upon the detectedsignal level. Finally, the method includes providing the gain controlsignal as an input to each of the amplifiers of the second amplificationmodule and controlling a gain of the second amplification module basedupon the provided gain control signal.

Features and advantages of the present invention include the integrationof an active splitter tuner arrangement onto a single IC, having arelatively small physical size and footprint, in order to drive multipletuners. This particular technique is also effective to improve secondand third order distortion product performance by using a differentialcircuit topology. The technique of the present invention also providesAGC for associated amplification modules by providing the gain controlfunction integrated with the amplification module. Finally, the presentinvention facilitates full use of the cable signal range with little orno noise figure penalty while also reducing space requirements, reducingcosts, and easing the manufacturability cable set-top boxes.

Other features and advantages of the present invention includecompletely autonomous local AGC of variable gain type amplifiers whichfacilitates maximizing the output SNR plus distortion ratio of theassociated tuners. An independent AGC function is included for eachoutput to cover both positive and negative cable roll-off. Currentsteering circuits are used in second stage amplification modules toswitch in or switch out gain smoothly with minimum impact on linearity.Techniques in accordance with the present invention also facilitateproviding signals proportional to amplifier gains and are scalable toaccommodate different numbers of multiple tuner applications withoutcreating a significant impact on broadband bandwidth. Finally, thepresent invention combines high linearity and low noise in adistribution amplifier for multiple tuner applications with a largevariable gain range.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated in and constitute partof the specification illustrate embodiments of the invention and,together with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the present invention. In the drawings:

FIG. 1 is a top level block diagram of a conventional passive splittertriple tuner solution;

FIG. 2 is a top level block diagram of a conventional implementation ofa triple tuner solution having a separate low noise amplifier (LNA) atthe input of a passive splitter;

FIG. 3 is a simplified block diagram of an exemplary system constructedand arranged in accordance with the present invention;

FIG. 4 is a detailed block diagram of another exemplary systemconstructed and arranged in accordance with the present invention;

FIG. 5A is a circuit level illustration of amplifiers used in the systemof FIG. 4;

FIG. 5B is an alternative embodiment of a resistor divider used in theamplifiers of FIG. 5A;

FIG. 6 is an illustration of the circuit of FIG. 5A at a maximum gainsetting;

FIG. 7A is an illustration of the circuit of FIG. 6 configured for areduced gain;

FIG. 7B is an illustration of a process for reducing gain in theexemplary embodiment of FIG. 7A;

FIG. 8A is an illustration of the circuit of FIG. 7A configured foradditional gain reductions;

FIG. 8B is an illustration of the circuit of FIG. 8A configured forminimum gain;

FIG. 9 is a circuit level diagram of one of the first stage amplifiersshown in the circuit of FIG. 5A;

FIG. 10 is a circuit level diagram of one of the second stage amplifiersshown in the circuit of FIG. 5A;

FIG. 11 is an alternative embodiment of the first stage amplifiers shownin FIG. 5A and configured to provide increased common-mode rejection;

FIG. 12A is a graphical illustration of a signal used to drive theexemplary arrangement of FIGS. 9 and 11;

FIG. 12B is a schematic diagram of a first portion of an exemplarycircuit for controlling voltage headroom of the circuit of FIGS. 9 and11;

FIG. 12C is a schematic diagram of a second portion of an exemplarycircuit for controlling headroom in the circuit of FIGS. 9 and 11;

FIG. 13 is an alternative embodiment of the circuit shown in FIG. 11;

FIG. 14 is a top level block diagram of an alternative embodiment of theexemplary system shown in FIG. 4;

FIG. 15 is a more detailed illustration of the alternative embodimentshown in FIG. 14;

FIG. 16A is a simplified exemplary schematic diagram of the first stageamplifiers in the illustration shown in FIG. 15;

FIG. 16B is a more detailed version of the illustration of FIG. 16A; and

FIG. 17 is an exemplary schematic diagram of the second stage amplifiersused in the illustration of FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the figures. Any actual software code with thespecialized controlled hardware to implement the present invention isnot limiting of the present invention. Thus, the operation and behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 provides a top level block diagram illustration of a conventionaltriple tuner solution. In FIG. 1, a triple tuner circuit 100 includes aninput port 102 and a passive splitter 106 configured to receive an inputsignal. The passive splitter 106 receives the input signal and splitsthe signal into three outputs that are used to drive matching tuners108, 110, and 112. Many of the conventional approaches position the LNAinside of the tuner itself, providing one IC or discrete solution withthe LNA inside.

The splitter 106 merely passively splits the input signal. In so doing,however, the splitter 106 creates insertion losses in accordance withthe expression 10 log₁₀ N, where N=the number of splitter outputs andloss is the amount of attenuation. In FIG. 1, for example, the actualloss appearing at the input of the tuners is about 4.77 dB. Next, thetuners 108, 110, and 112 are respectively coupled to the demodulators114, 116, and 118. In practice, the tuners 108, 110, and 112 aretypically used to derive signals of different frequency bands from theinput signal received at the input port 102. The demodulators areprovided to demodulate the associated derived signals and extractentertainment programming information therefrom. The demodulators 114,116, and 118 include respective automatic gain control AGC paths 122,124, and 126 to control the gain of the circuit 100.

As noted above, the conventional tuner solutions, such as the system100, include passive splitters which create insertion losses thatdegrade the performance of the downstream tuners 108, 100, and 112. Theinsertion losses introduced by the splitter 106, for example degradesignal quality, which in-turn, directly impacts the noise figure,distortion performance, and SNR of associated system amplifiers.

FIG. 2 illustrates another conventional tuner system 200 having improvedperformance over the system 100 of FIG. 1. In FIG. 2, the tuner system200 includes the passive splitter 106 used in the system 100 but alsoincludes a fixed gain control LNA 204. The fixed gain LNA 204 helps toavoid the noise figure problems discussed above in association with thetuner system of FIG. 1. Because the gain of the LNA 204 is often on theorder of 15 dB, fixed attenuators 206, 208, and 210 are used on theoutput to reduce the signal level to each tuner to a more reasonablevalue.

For example, assume the gain of the LNA 204 is 15 dB and its noisefigure is 3.5 dB. Assume also that the noise figure of each of thetuners 108, 110, and 112 is about 8 dB. If the total loss from the inputof the power splitter 106 to the output of each of the attenuators 206,208, and 210 is 13.1 dB, the noise figure at the output of each of thetuners remains at 8 dB—no degradation. However, the gain has beenincreased by 1.9 dB, making the signal level at the output of each tuneralmost 2 dB higher. This will result in more distortion.

Instead, assume the combined loss of the splitter 106 and attenuators206, 208, and 210 is equal to 15 dB. Now each of the tuners 108, 110,and 112 sees an input signal at the same level as that seen at the inputof the LNA 204. This results in a noise figure of 9.3 dB at the outputof each tuner—an increase of 1.3 dB. Therefore, although the tunersystem 200 provides improvements over the system 100 by incorporatingthe fixed gain LNA 204 in front of the splitter 106, the noise figure ordistortion of the system can still be degraded.

FIG. 3 is a top-level block diagram of an exemplary splitter amplifier300 constructed and arranged in accordance with the present invention.More importantly, the exemplary splitter amplifier 300 provides asolution to the noise figure and distortion problems of FIGS. 1 and 2.In FIG. 3, the fixed gain LNA 204 of FIG. 2 has been replaced by anautonomous, variable gain LNA 302. A power splitter 306 distributes thesignal output from the LNA 302 along multiple signal paths. A powerdetector 304 monitors the output of the LNA 302 to maintain signals thatare output therefrom at a constant level.

Specifically, the power detector 304 is configured to receive athreshold level input such that signals output from the LNA 302 that areabove a predetermined threshold value will be maintained at a constantlevel. In other words the gain automatically adjusts itself tocompensate for variations in the input signal level so that each of thetuners 108, 110, and 112 receives a signal having a constant level atits input when signal output from the LNA 302 are above thepredetermined threshold value. If signals are below the predeterminedthreshold value, the level of signals received as inputs to the tunerswould correspondingly drop. The approach of FIG. 3 insures that the LNA302 achieves good noise figure performance for low-level signals, whileat the same time, achieves good distortion performance for high-levelsignals.

An additional feature of the splitter amplifier 300 is that itspecifically positions the LNA 302 outside of the tuners, which helpsimprove the dynamic range of the signal path. Many of the conventionalapproaches position the LNA inside of the tuner itself, providing one ICor discrete solution with the LNA inside. Here, the LNAs 308, 310, and312 are optionally separated from respective tuners 108, 110, and112—optional if the LNA is not included in the tuner function. Theinventors of the instant invention have discovered, however, that bykeeping the LNA separate from the tuners that, among other things,multiple AGC paths can be accommodated without compromising noise figureor distortion.

FIG. 4 is a more integrated version of the splitter amplifier 300 ofFIG. 3 which maintains the LNA separate from the tuners and incorporatesmultiple AGC paths. In FIG. 4, a tuner system 400 includes an input port402 configured to receive an input programming material signal. Althoughthe input signal can be of the single-ended or differential variety,most programming type input signals are single-ended becausesingle-ended signals are easier to provide. Thus, for purposes ofillustration, the input signal format discussed in the instant inventionwill be based upon a received single-ended signal.

A mechanism 404 is provided to receive the single-ended input signal andconvert it into a differential signal. Although FIG. 4 depicts themechanism 404 as a balance/unbalance (balun) device, any single-ended todifferential signal conversion mechanism or technique can be used. Thus,in the system 400, the balun 404 receives the single-ended signal at theinput port 402 and converts it into a differential signal provided tobalun output ports 404 a and 404 b. In an alternative embodiment, theoutput 404 b can be connected to an AC ground, thus converting the inputsignal to a single-ended signal.

The output differential signal is then provided as inputs to an activesplitter amplifier arrangement 406. In addition to noise immunity andcommon-mode rejection properties inherent in differential input signals,the inventors of the present invention have discovered throughexperimentation that differential input signals also reduce second orderand third order composite distortion products.

Another benefit of using differential signals in active integratedsolutions, such as the amplifier splitter 406, is the ability tocapitalize on the matching properties offered by ICs. The activesplitter 406 includes these matching properties because the low noiseamplification function and the splitter function are integrated into asingle IC, eliminating the requirement for signals to be passed orcoupled from an LNA IC to another IC containing a splitter. Asunderstood in the art, potentially significant levels of noise areintroduced into signals each time they are passed from one IC toanother. In differential signals, however, any noise is eliminated sinceonly the difference between the positive and negative signal componentsare amplified.

Third order terms of composite distortion products are minimized becausethe signal swing across each transistor of the associated differentialtopology is reduced. Second order distortion is further reduced due tothe balanced nature and symmetry of differential input signals. Thisinherent benefit of differential amplifiers, for example, reduces thirdorder distortion products. The differential topology approach similarlyprovides significant improvements in the reduction of the second orderterms of composite distortion products.

The exemplary embodiment of the present invention illustrated in FIG. 4,therefore, provides a single active amplifier splitter 406, which can bea low noise variable gain amplifier (LNVGA). This single LNVGA performsboth the LNA and splitter functions for the tuners shown in the systems100 and 200 of FIGS. 1 and 2, respectively. The splitter 406 includes afirst stage differential amplification module 408, which in-turnincludes respective inverting and non-inverting input ports 410 and 412.The amplification module 408 also includes respective inverting andnon-inverting output ports 414 and 416. The module 408 provides firststage amplification for a differential input signal received at theinput ports 410 and 412 and provides the corresponding differentialoutput signal to a number of second stage amplifiers 418, 420, and 422.

The second stage amplifiers 418, 420, and 422 of the amplifier splitter406 are integrated with the splitter function. Whereas, the amplifiers302, 308, 310, and 312 in the conventional system 300 of FIG. 3, arephysically separate from the splitter 306. The LNAs being separated fromthe splitter, as implemented in the conventional system 300, creates theneed to maintain at least a 75 ohm impedance transmission line system.Integrating the function of the splitter and the LNA, as accomplished inthe present invention, enables system designers to simply parallel theamplifiers without the need of maintaining 75 ohm impedance lines.Although three second stage amplifiers are discussed herein for purposesof illustration, any number of amplifiers can be used to accommodateparticular user and/or system requirements.

The second stage amplifiers 418, 420, and 422 provide differentiallyamplified signals along differential signals transmission lines 424,426, and 428 to respective tuners 430, 460, and 470. Providingdifferential signals as inputs to the tuners 430, 460, and 470 helps tosuppress any noise generated between the tuners and the LNAs. Providingthe differential transmission lines 424, 426, and 428 also inherentlyprovides higher impedance transmission lines. Specifically, thedifferential lines provide about 200 ohms of differential impedance,which helps maintain the overall power consumption.

Since each of the tuners 430, 460, and 470 along the signal paths 424,426, and 428 are substantially identical, the discussion of the tuner430 will apply equally to the tuners 460 and 470. Furthermore, althougha specific tuner implementation is shown in FIG. 4, numerous other tunerapproaches can be used. In the exemplary system 400, after thedifferential signal is amplified, it's provided as an input to a mixer432. The mixer 432 mixes the amplified signal up to a predeterminedfrequency based upon a signal provided by a local oscillator 434. Asnoted above, the tuners 430, 460, and 470 do not include LNAs, whichhelps to reduce the dynamic range of the associated signal path.

Next, the signal is provided off-chip to a high intermediate frequency(HI-IF) filter 436. In one embodiment, a surface acoustic wave (SAW)filter is used because of its very narrow bandwidth and flat group delayproperties. However, any suitable narrow-band filter arrangement (e.g.,filters having five or fewer channels) can be used. The differentialsignal is then provided to another mixer 438 that mixes the signal downbased upon a signal provided by another local oscillator 440. A lowfrequency VGA 442 receives the signal output from the mixer 438,provides a final level of amplification, and then provides the signal toa standard intermediate frequency (IF) filter 444. The standard IFfilter 444 could be a SAW or a lumped element (LC) filter. The filteredsignal is then provided to a demodulator 446.

The demodulator 446 can be, for example, a set-top box chip, a cablemodem, or a PVR, etc. In the system 400, the demodulator 446 not onlydemodulates the differential signal processed along the path 424, but italso controls the gain of an output stage of the tuner 430.Additionally, the demodulator 446 controls the gain of a correspondingoutput buffer (not shown), and controls the gain of the second stageamplifiers 418, 420, and 422.

The demodulator 446 controls a gain of the tuner 430 by monitoring thequality and level of signals output from the tuner 430 and providing acontrol signal to the VGA 442 along a feed back path 448. The controlsignal is used to adjust the gain of the VGA 442 in accordance with thequality of a particular channel selected by the user. The demodulator446 controls the gain of its corresponding output buffer to furtheroptimize the SNR of the signal associated with the selected cablechannel, thus compensating for system cable losses.

At a broader level, the demodulators, including the demodulator 446,control the gain of the second stage amplifiers 418, 420, and 422 inorder to compensate for different modulationschemes. Here, for example,the demodulator 446 provides a gain control signal 451 to an AGC controldevice (not shown). The AGC control device is included in the outputbuffer and in-turn, provides the gain control signal 451 to the secondstage amplifier 418 to provide gain control thereto. Similarly, thedemodulators along the signal paths 426 and 428 provide respective gaincontrol signals 452 and 453 to the second stage amplifiers 420 and 422respectively. Alternatively, the demodulators can be configured todirectly control the gain of the second stage amplifiers 418, 420, and422 via analog signals output from each of the demodulators.

Other implementations of the tuners as well as the AGC control functionsare possible. In alternative embodiments, the amplifiers 418, 420, and422 can be controlled digitally via an inter-integrated (I²C)circuit-like digital interface 480. Other exemplary implementations ofthe AGC control function include the use of arrangements such as anexternal voltage. A specific alternative embodiment of the amplifier 406and the AGC control feature will be discussed in greater detail below.

In the embodiment of FIG. 4, gain in the first stage amplificationmodule 408, and thus to an extent, the overall gain of the activeamplifier splitter 406, is controlled by an AGC block 454. The AGC block454 can be, for example, a power detector that monitors the differentialsignal output from the amplification module 408 to determine the amountof gain needed. Specifically, the level of the module 408 output signalis compared with a predetermined threshold signal produced via aresistor 455. Since the resistor 455 is not within the module 408, thepredetermined AGC threshold can be adjusted externally. A control signal456 is produced and forwarded to an input of the amplification module408 along a feedback path in order to adjust its input in accordancewith the threshold comparison.

FIG. 5A provides a more detailed illustration of the first stageamplification module 408 and the second stage amplifiers 418, 420, and422. In FIG. 5A, a multistage amplification arrangement 500 includes afirst stage section 502 and a second stage section 504. The first stagesection 502 includes the amplification module 408 coupled toinductor/resistor devices 508 a and 508 b. A specific function of theamplification module 408 is to provide low noise gain for the receivedinput signal and provide overall dominant AGC for the entire system 400,as discussed above. The inductor/resistor devices 508 a and 508 b areused to provide, for example, bandwidth enhancements when driving aparasitic capacitance load. The resistive part of the inductor/resistordevices 508 a and 508 b are also used to set the voltage gain. Althoughany suitable device can be used, for purposes of illustration, the loaddevices 508 a and 508 b are formed of a series resistor-inductorcombination load.

Each inductor/resistor device 508 a and 508 b respectively includesresistors 510 a and 510 b, and inductors 512 a and 512 b. Additionally,inductors 514 a and 514 b, used as radio frequency (RF) chokes, arerespectively connected across the resistor combination loads 512 a/510 aand 512 b/510 b. In other words, the inductors 514 a and 514 b areessentially a DC short but present a high impedance in the particularfrequency band of interest. One terminal of the load device 508 a iscoupled to an output terminal 414 and one terminal of the load device508 b is coupled to an output terminal 416 of the amplification module408. The load devices and inductors can be off-chip in both the firstand second stage sections 502 and 504 respectively.

The inductors 512 a and 512 b are series peaking inductors. Thecorresponding resistors 510 a and 510 b act as a resistive load, withthe respective inductors 512 a and 512 b acting to flatten out thebandwidth. The inductors 514 a and 514 b are provided as a means toachieve a DC short in order to obtain voltages on the output terminals414 and 416 as high or as close to V_(DD) as possible in order tomaximize distortion performance in the first stage section 502.

The second stage section 504 includes the amplifier 418 havinginductor/resistor devices 518 a and 518 b connected across itsdifferential output ports. The other amplifiers 420 and 422 includeinductor/resistor devices 520 a/520 b and 522 a/522 b connected andfunctioning in the same manner discussed above in relation to the firststage section 408. That is, the inductor/resistor devices of the secondstage section 504 operate to maximize its distortion performance. Theoperation of the amplification module 500 will now be discussed ingreater detail.

The amplification module 408, as indicated above, includes thedifferential input ports 410 and 412 and is comprised of two groups of10 VGAs 531 through 550 connected in parallel. As shown in FIG. 5A, eachof the amplifiers 531 through 550 includes differential input ports. Theinverting and non-inverting input leads of the differential amplifiers531 through 550 are connected to the input ports 410 and 412respectively. In the first stage section 502, signals provided as inputsto the amplifiers 531 through 540 are unattenuated. On the other hand,signals input to the amplifiers 541 through 550 are attenuated by apredetermined amount via an attenuator 552. In the instant embodiment,the attenuator 552 is a resistor divider that provides about 15 dB ofattenuation. FIG. 5B, however, is an illustration of an alternativeembodiment 552 a of the attenuator 552. Although two groups ofamplifiers are described in FIG. 5A, any number of amplifiers can beused in order to achieve desirable system performance requirements.

In FIG. 5B, the alternative attenuator 552 a is constructed by dividingthe attenuator 552 of FIG. 5A into multiple successive portions 560,each including a resistor network 562 having an amplifier 564 connectedacross its output. The portion 560 is differentially connected to theother portions of the attenuator 552 a. That is, the differential inputports of one portion of the attenuator 552 a are respectively connectedto the differential input ports of a successive portion of theattenuator 552 a. Each of the portions 560 provides about 1.5 dB ofattenuation. The total attenuation of the attenuator 552 a is thereforeabout 15 dB—substantially the same as the attenuator 552 of FIG. 5A. InFIG. 5B, however, the amplifiers 564 can be selectively activated toprovided a means of tapping particular sections of the attenuator 552 ato provide a greater and finer number of attenuation settings on the waydown the chain of portions 560. The attenuator 552 a, by dividing theattenuator 552 into multiple sections, provides for better overalldistortion performance in intermediate gain settings.

Returning to FIG. 5A, the second stage section 504 provides a pluralityof outputs to drive each of the tuners 430. Gain control provided by thedemodulator 446 assists in fine tuning the level of individual signalsselected when the user changes or tunes to a particular channel.

During operation of the first stage section 502, only 11 adjacentamplifiers in the amplification module 408 are powered-up to achieve aparticular gain setting. FIG. 6 illustrates an exemplary maximum gainsetting mode of the first stage amplification module 408. In order toset the module 408 to its maximum gain setting, the amplifiers 531through 540 are each set to their maximum gain level and the amplifier541 is set to a minimum gain level. The remaining amplifiers 542 through550 are completely powered-off.

In a third alternative embodiment, the amplifiers 541-550 may be omittedand ten more section 560s may be added to FIG. 5B

The overall gain of the amplification module 408 is reduced bydecreasing the gain of the amplifier 531 and simultaneously increasingthe gain of the amplifier 541 until the amplifier 531 is at its minimumgain level and the gain of amplifier 541 is at its maximum gain level.After minimum gain is reached in the amplifier 531, it is completelypowered-off. The amplifier 531 must be completely powered off because ifit remains powered up, even at a minimum gain value, it will introduceparasitic feed-through which will reduce the overall linearityperformance of the amplification module 408.

FIG. 6 illustrates the process of powering the amplifiers 531-550 up anddown. As noted above, the gain of the amplifier 531 is decreased whilethe gain of the amplifier 541 is simultaneously increased in order todecrease the overall gain of the amplification module 408. In otherwords, the amplifiers 531 and 541 are operating at the same time but atopposite polarities. This process or requiring simultaneous operationinsures that the gain of the amplifiers can go smoothly from a highervalue to a lower value and vice versa without first wiggling up and/ordown. After the amplifier 531 is faded out and its output is no longercontributing to the amplification module 408, it is completely powereddown. This process continues (i.e., additional amplifiers being poweredup and down), until the amplification module 408 reaches its minimumgain setting.

FIG. 7A also illustrates a further reduction of the overall gain of theamplification module 408. In the module 408, as the amplifier 531 isturned completely off, the amplifier 542 is powered on at a minimum gainvalue. The gain of the amplifier 532 can then be decreased and the gainof the amplifier 542 can be increased until the amplifier 532 is at itsminimum gain value and the amplifier 542 is at its maximum gain value.FIG. 7B more clearly illustrates the process of reducing the gain of theamplification module 408 from a maximum setting to a minimum setting.

In FIG. 7B, the rows 700-708 represent progression of the process ofdecreasing gain in the amplification module 408 by simultaneouslypowering down and powering up corresponding amplifiers. The row 700depicts 10 non-attenuated amplifiers at maximum gain. As the gain in theamplification module 408 begins to decrease in row 702, the amplifier531 begins to power down as the amplifier 541 simultaneously begins topower up. In the row 704, the amplifier 531 is completely turned off atthe moment the amplifier 541 is completely turned on. Next, theamplifier 532 begins to power down as the amplifier 542 simultaneouslybegins to power up, as indicated in the row 706. Finally, the amplifier532 is completely turned off at the moment the amplifier 542 iscompletely turned on.

The gain of the amplification module 408 is further reduced as thisprocess is continued until all of the amplifiers 531 through 540 arepowered-off, and the amplifier 541 through 550 are set to their maximumgain value. In this mode, which is illustrated in FIG. 8A, the overallgain of the amplification module 408 is now about 15 dB below themaximum gain setting illustrated in FIG. 6.

To achieve an additional gain reduction, the gain of the amplifier 541is reduced until it reaches minimum gain. After its minimum gain settingis achieved, the amplifier 541 is completely turned off. This process ofturning off the amplifiers after the minimum gain has been achieved, iscontinued through the remainder of the amplifiers 542 through 550 untilonly a minimum number of amplifiers remain on.

FIG. 8B illustrates an exemplary case where the amplifiers 549 and 550are left on at their maximum gain value settings in order to achieve anoverall minimum gain value condition for the amplification module 408.Although the illustration of FIG. 8B depicts the use of two amplifiersremaining on at their maximum gain, any suitable number of amplifiers,including zero, can be used. The inventors of the present invention havediscovered through experimentation, however, that 1-3 amplifiersremaining on is an acceptable number to achieve optimal operation of theamplification module 408 at its minimum gain level. The minimum gain ofthe amplification module 408 is balanced against the minimum number ofpowered-on amplifiers needed to maintain acceptable bandwidth flatnessand linearity characteristics. If more amplifiers are turned off thanthe minimum limit, more gain reduction is achieved but the parasiticcapacitance produces frequency feed-through and degrades the bandwidthflatness and overall linearity performance of the amplification module408.

FIG. 9 is an illustration of an exemplary circuit diagram that can beconfigured for use as the amplifiers 531 through 550 shown in FIGS.5A-8B. In FIG. 9, an amplifier circuit 900 includes input active devices902 and 904. Although other active device types can be used, the activedevices in the embodiment of FIG. 9 are implemented using one or moretransistors, which can be field effect transistors (FETs) or bi-polardevices.

Next, a transistor pair including active devices 906 and 909 is alsoprovided in FIG. 9, with the device 906 acting as a cascode for thedevice 902. Similarly, another transistor pair includes transistordevices 910 and 912, where the transistor device 912 acts as a cascodefor the device 904. Also included in the circuit 900 are respectiveinverting and non-inverting drive voltage input ports 914 and 916 aswell as inverting and non-inverting AGC drive voltage input ports 918and 920, respectively. Finally, inverting and non-inverting output ports922 and 924 are also provided.

In the circuit 900, the maximum gain occurs when a maximum AGC drivevoltage is applied to the input terminal 920 and a minimum AGC drivevoltage is applied to the input terminal 918. In other words, the amountof signal permitted to go through the devices 906 and 912 can be varied,based on the signal swing across the input ports 918 and 920 inaccordance with the applied drive voltage signals. Thus, for example, inthe case of the input device 902 the output current can be steeredeither partially or completely through the device 906 or the device 909based upon the AGC drive voltage across the input ports 918 and 920.

Therefore, if a gate voltage of the device 906 is higher than a gatevoltage of the device 909, more of the current will be steered throughthe device 906 to the output port 922. If the AGC drive voltage iscompletely tilted such that the device 906 is completely on and thedevice 909 is completely deactivated, then all of the output signal willgo through to the inverting output port 922. This mode is representativeof the circuit 900 configured for a maximum gain value setting. Oneadvantage of this technique is that the linearity of the circuit 900 isgreatly improved despite the presence of gain reducing signals such assecond and third order inter-modulation products.

FIG. 10 illustrates an exemplary circuit 1000 that can be used as thesecond stage amplifiers 418, 420, and 422. Since the noise performanceof the second stage amplifiers 418, 420 and 422 is not as critical asthe noise performance of the amplification module 408, a much moresimplified circuit arrangement can be utilized. The circuit 1000includes input transistor devices 1002 and 1004 for receiving inverting,and non-inverting input drive signals via inverting and non-invertinginput ports 1006 and 1008, respectively.

The input devices 1002 and 1004 form a differential pair amplifier whichincludes devices 1010, 1012, 1014, and 1016 provided for controllinggain, and a source degeneration mechanism 1020. As a voltage level of anAGC voltage signal received via an AGC drive voltage input terminal 1018is increased, channel resistance of the devices 1010, 1012, 1014, and1016 is decreased by the degeneration mechanism 1020 and the gain of thecircuit 1000 is thereby increased. The amplifiers 1010 through 1016 areat a minimum gain when the received AGC drive voltage is zero volts anda maximum gain when the AGC drive voltage is substantially equal to thecircuit supply voltage V_(DD).

An advantage of the circuit 1000 is that it provides a more simplifiedapproach for varying amplifier gain. Additionally, the devices 1010-1016provide better linearity because the signal swing across any one deviceis reduced. Active device such as FETs, operate as linear resistors onlywhen the drain source voltage is zero. Thus, any signal present willappear at the sources of the input devices 1002 and 1004, andsubsequently across the devices 1010-1016. By using a large number ofFETs in series, any signal present can be broken into smaller segments.The resistors at the gates of the devices 1010-1016 are high-valueresistors, essentially making the gates floating, in terms of AC. Theresistors within the degeneration device 1020 are used to keep a limitedvoltage across the devices 1010-1016. In so doing, the circuit 1000 isproviding a limited AGC range. The range is limited on one end, forexample, when all of the devices 1010-1016 are off, the resistors 1020keeps the voltage across the devices 1010-1016 reduced.

In the circuit 1000, when the gates of the transistor devices 1010-1016are activated by the AGC drive signal, all of the transistors 1010-1016are turned on, creating a variable resistance. As the voltage level ofthe AGC drive signal is increased, the resistance across the devices1010-1016 becomes smaller, producing a maximum gain in the amplifiercircuit 1000.

The circuit arrangement 1000 improves linearity but does not maintainconstant linearity because its linearity varies as the resistance acrossthe device 1020 varies. This effect hurts the overall noise anddistortion performance of the circuit arrangement 1000. However, thecircuit 1000 is acceptable for the purposes for which it is used in thepresent invention because the noise performance of the amplifiers 418,420, and 422 is not as important as the noise performance of theamplification module 408. Therefore, although the circuit 1000 issuitable for use in the second stage amplifiers 418, 420, and 422, it isnot desirable for use in the first stage amplification module 408.

FIG. 11 provides an alternative arrangement to the amplifier circuit 900illustrated in FIG. 9. FIG. 11 is a combination of the current steeringtechnique discussed in relation to FIG. 9, and the active degenerationtechnique discussed in relation to FIG. 10. In FIG. 11, a circuit 1100includes respective inverting and non-inverting input ports 1102 and1104 for receiving respective non-inverting and inverting controlvoltages.

Also included in the circuit 1100 is an AGC voltage input port 1106 andtransistor devices 1108 and 1109, which are cooperatively used toprovide source degeneration. In FIG. 11, the inverting and non-invertingcontrol voltages received via the input ports 1102 and 1104, control thegain of the circuit 1100 in a manner similar to the current steeringtechnique discussed above. Also, the AGC voltage received via the inputport 1106 controls the process of active source degeneration associatedwith the devices 1108 and 1109. Respective inverting and non-invertingdifferential input ports 1110 and 1111 are configured to receive aninput differential signal. Resistors 1112, 1114, and 1116 limit theextent of the source degeneration. Finally, inductors 1118 and 1120 areprovided to reduce common-mode gain of the circuit 1100 and to reduce DCvoltage associated with the source degeneration process. In summary, thecircuit 1100 provides an alternative arrangement that has bettercommon-mode rejection than the circuit 900 illustrated in FIG. 9.

One goal with the circuit 1100 is to maintain a maximum amount ofvoltage across lower differential transistor pair 1130 and 1132. This isachieved in the circuit 1100, for example, by keeping voltages at sourcenodes n1 and n2, as high as possible over the AGC control range,preferably near V_(DD). This arrangement helps to maintain desirablenoise and linearity performance within the circuit 1100. To keep thevoltages at source nodes n1 and n2 high, the gate voltages +V_(CTRL) and−V_(CTRL) must also be kept as high as possible. With one of the gatevoltages +V_(CTRL) or −V_(CTRL) high, the other can be cranked low. Forexample, if +V_(CTRL) is held near V_(DD), then −V_(CTRL) can be crankedlower with the voltage at n1 and n2 being kept at a high value. Thevoltages at n1 and n2 will basically be close to the value of thehighest of +V_(CTRL) and −V_(CTRL), minus about ½ to ⅘ths of a volt. Thekey to the circuit 1100, is to keep at least one of the voltages+V_(CTRL) or −V_(CTRL), high. FIG. 12A graphically depicts the desirabledrive voltage necessary for maintaining proper operation of the circuits900 and 1100.

In FIG. 12A, the solid and dotted lines are representative of +V_(CTRL)(positive) and −V_(CTRL) (negative), respectively. The horizontal line1222 depicts a magnitude of the +AGC and −AGC voltages. The verticalline 1224 depicts a magnitude of the +/−V_(CTRL) voltages. As shown inthe graph, when the AGC voltage, along the axis 1222 is at its minimumvalue, the −V_(CTRL) is much lower than +V_(CTRL), with +V_(CTRL) beingequal to V_(DD), along the axis 1224. When this happens, transistors1134 and 1136 of FIG. 11 are fully turned on, and transistors 1138 and1140 are fully turned off. Therefore, all the current will flow into theload from the transistors 1130 and 1132. As the AGC voltage is changedalong the axis 1222, the −V_(CTRL) voltage begins to increase, while+V_(CTRL) remains the same, sitting near V_(DD). When the AGC voltagereaches a middle level, +V_(CTRL) and −V_(CTRL) are equal, thus forminga cross-over point 1226, which is slightly lower than V_(DD). In otherwords, the cross-over point 1226 formed by an intersection of +V_(CTRL)and −V_(CTRL) is close to V_(DD). This keeps the voltage at the nodes n1and n2 as high as possible.

FIG. 12B provides an illustration of a first portion 1200 of anexemplary circuit arrangement used to provide the AGC drive voltagesignal used in the current steering circuit 900 of FIG. 9 and thecircuit 1100 of FIG. 11. FIG. 12C presents a second portion 1218 of theexemplary circuit arrangement. In particular, the first and secondcircuit portions 1200 and 1218 cooperatively provide the ability tomaximize headroom in the device 900 by controlling its input activedevices 902 and 904. Headroom maximization is desirable for achievingsufficient linearity in the circuit 900. Headroom and linearity can bemaximized by maintaining a large drain source voltage across the devices902 and 904 during operation.

A large drain-source voltage across the devices 902 and 904 is achieved,for example, by insuring that its cascode, active device 909, iscompletely on before beginning to deactivate the active device 906.Activation in this manner prevents the lowering of the common-sourcevoltage of the devices 906 and 909, which is the drain of the device902. Lowering the common-source voltage will reduce the headroom andjeopardize the linearity performance of the device 902. The first andsecond portions 1200 and 1218 are configured to provide a drive signalat the inputs 918 and 920 of the circuit 900 that is capable offacilitating the desired headroom and linearity characteristics.

More particularly, the exemplary circuit 1200 of FIG. 12B is configuredto provide differential output signals having offset voltages. Thedifferential output signals are then provided as inputs to the secondcircuit portion 1218 of FIG. 12C. FIG. 12B includes a differentialamplifier with split resistor loads having differential input ports 1207and 1209 configured to receive a differential AGC bias signal. Thecircuit 1200 is also configured to receive an input supply voltagesignal V_(DD). Two pairs of skewed differential output ports 1204/1208and 1206/1210 are provided and match the input ports depicted in thecircuit 1218 of FIG. 12C.

The circuit 1200 produces a first differential output signal at theoutput ports 1204 and 1208 and a second differential output signal atthe output ports 1206 and 1210. The first and second differential outputsignals have different voltage levels and are configured to drive thecircuit 1218. The inventors of the present application have discoveredthat a voltage difference within a range of about 1.5 to 2.5 volts issuitable. Thus, during operation, the circuit 1200 provides the firstand second differential output signals, that are also responsive to thesupply voltage signal V_(DD), as inputs to drive the circuit 1218.

The circuit 1218 of FIG. 12C includes two differential pair amplifiercircuits 1202 and 1203 and differential input ports 1204, 1206, 1208,and 1210. Also included in the circuit 1218 are inverting andnon-inverting AGC drive signal output ports 1214 and 1216 configured toprovide the AGC drive signal for input to the current steering circuit900. The circuit 1200 is energized by the supply voltage signal V_(DD).During operation, the first differential output signal is provided tothe differential input ports 1204 and 1208 of the active devices 1202and 1203 respectively. The second differential output signal is providedto the differential input ports 1210 and 1206 of the active devices 1202and 1203 respectively. In response, the circuit 1218 produces adifferential output drive signal V_(AGC) at the differential outputports 1214 and 1216.

Careful adjustments can be made to the differential output drive signalV_(AGC) and/or to the differential input AGC bias signal to provide asignal having a magnitude and waveform characteristics appropriate formaximizing headroom and linearity in the circuit 900. FIG. 12Agraphically illustrates a desirable relationship between magnitudes ofthe supply voltage signal V_(DD), the input AGC bias signal, and theoutput drive signal V_(AGC) for maximizing headroom and linearity.

FIG. 13 illustrates a circuit 1300 that is an alternative embodiment ofthe circuit arrangement 1100 shown in FIG. 11. The circuit 1300 providesfurther improvements in the second order distortion characteristics overthe circuit 1100. Improved second order distortion is key to achievingoptimal cable tuner LNA performance. In FIG. 13, the circuit 1300includes an RF choke 1302 used in a common-source arrangement to reducecommon mode gain while, at the same time, not reducing the DC voltageheadroom across the other active circuit devices. In a preferredembodiment of the present invention, the RF choke 1302 has a large ACimpedance and zero DC impedance. The RF choke 1302 is also an off-chipcomponent due to the technical challenges associated with fabricatingon-chip RF chokes.

FIG. 14 is an alternative embodiment of the circuit 300 shown in FIG. 3and includes a circuit arrangement 1400. The arrangement 1400 includesan amplification module 1402 with a completely autonomous andindependent AGC function. Independent and autonomous AGC provides theamplification module 1402 with the ability to maximize the system's SNR.Stating that the AGC function is autonomous means that the AGC functionis controlled completely and independently within the amplificationmodule 1402. In the circuit arrangement 400 of FIG. 13, for example, thegain of the second stage amplifier 418 is controlled by the demodulator450. As discussed in relation to FIG. 4, the amplification module 1402can be, for example, an LNVGA.

In FIG. 14, the amplification module 1402 includes a first stage device1404, a self-contained AGC mechanism 1406, and second stage amplifiers1408, 1410, and 1412. The AGC mechanism 1406 facilitates autonomous AGCcontrol across the entire spectrum of set-top boxes and cable modems, asan example. The AGC mechanism 1406 is configured to produce a constantlevel for the output signal to each tuner as opposed to having the gaincontrolled by some other down-stream device. In this manner, the SNR ofsignals can be maximized to each tuner from the amplification module1402.

An AGC feedback path 1409 is provided to monitor an output of one of thesecond stage devices 1408, 1410, and 1412. An AGC loop 1414 is alsoprovided to facilitate the AGC adjustments to all of the second stagedevices 1408, 1410, and 1412 based upon the monitoring of the onedevice, which is depicted as the device 1408 in FIG. 14. Alternatively,an output of the AGC mechanism 1406 could also be fed to the first stagedevice 1404 to provide AGC adjustments thereto. Finally, a demodulator1416, functionally similar to the demodulator 4346 of FIG. 13, isprovided for demodulation of the input signals. Unlike the demodulatorof FIG. 4, however, the demodulator 1416 does not provide any AGCcontrol. FIG. 15 provides a more detailed illustration of theamplification module 1402.

In FIG. 15 the amplification module 1402 includes power detectorcircuits 1500 (optional) and 1501 for monitoring power levels of theamplification module 1402. Although both of the detector circuits 1500and 1501 are shown in the exemplary embodiment of FIG. 15, adequatepower level monitoring can be achieved by using only one of thedetectors. The first stage device 1404 of the amplification module 1402includes differential amplifier 1502 having respective non-inverting andinverting output ports 1503A and 1503B and an amplifier 1504 havingnon-inverting and inverting output ports 1505A and 1505B, respectively.The amplification module 1404 via the amplifiers 1502 and 1504 providesa first stage of amplification for the amplification module 1402,wherein the differential amplifier 1502 is used as a high gain amplifierand the differential amplifier 1504 is used as a low gain amplifier.

The high gain amplifier 1502 receives an unattenuated input signaldirectly from the off-chip balun 404. The amplifier 1504 receives thesame input signal. However, the input signal to the amplifier 1504 isattenuated by a differential ladder 1506, which includes impedancedevices 1507-1510. Although, in the exemplary embodiment of FIG. 15, theimpedance devices 1507-1510 are shown to be resistors, any suitableimpedance device can be used. The differential ladder 1506 reduces thegain contribution of the first stage amplifier 1504. A capacitor 1512provides a differential ac path to ground to maintain differentialbalance between the differential amplifiers 1502 and 1504.

Also included in the amplification module 1402 are the second stageamplification modules 1408, 1410, and 1412. Each of the second stagemodules 1408, 1410, and 1412 includes two differential amplifiers 1520and 1522 which form a second amplification stage. The amplifiers 1520and 1522 are configured to have their outputs summed by a summing device1521 which could be, for example, a wired-OR device. In the presentinvention, the amplifiers 1520 and 1522 are VGAs, although any suitableamplifier can be used. Additionally, as in the case of the system 400discussed above with reference to FIG. 4, three second stageamplification modules are used in the present invention, although anysuitable number of amplification modules can be used.

Each of the second stage 1520 of the modules 1408, 1410, and 1412 isdriven by the high gain amplifier 1502 of the first stage device 1404.Similarly, each of the second stage amplifiers 1522 is driven by the lowgain amplifier 1504 of the first stage device 1404. The gain of thesecond stage amplifiers 1520 and 1522 can be independently controlled,and the amplifiers are used to selectively combine signals output fromthe amplifiers 1502 and 1504. In other words, the second stageamplifiers 1520 and 1522 can be used to switch in or out the first stageamplifiers 1502 and 1504. This feature provides the amplification module1402 with the ability to vary its gain smoothly without effecting itsoverall linearity.

Maximum gain in the amplification module 1402 is achieved when theamplifier 1520 is set to a maximum gain value and the amplifier 1522 isset to a minimum gain value. It should be noted that the minimum gainsetting of the amplifiers 1520 and 1522 is actually a large negativegain, or in other words, a large amount of signal attenuation such thatthe amplifiers 1520 and 1522 become electrically insignificant tooperation of the amplification module 1402. The gain of theamplification module 1402 is decreased by reducing the gain of theamplifier 1520 while increasing the gain of the amplifier 1522. Thisprocess continues until the amplifier 1520 has reached a minimum gainvalue and the amplifier 1522 has achieved its maximum gain value. Afterthe amplifier 1520 has reached its minimum gain value, then theamplifier 1502 of the amplification module 1404 can be deactivated tokeep the input devices of the amplifier 1502 from further distorting theinput signal.

The gain of the amplification module 1402 can then be further attenuatedby reducing the gain of the amplifier 1522. The minimum gain setting ofthe amplifier 1522 is limited to a finite programmable level when theamplifier 1520 is set to its minimum gain. If the amplifier 1522 wasallowed to reach its minimum gain value, then no signal would passthrough to downstage tuners. This process is described more fully in thedescriptions of FIGS. 5A-8B above.

The amount of attenuation provided by the differential ladder 1506 isfundamentally important and determines a takeover point at which theamplifier 1502 is deactivated or switched out of the signal chain by theamplifier 1520. The selection of this takeover point balances thelinearity requirements of the amplifier 1502 with that of the amplifier1504. Therefore, the attenuation of the differential ladder 1506 and thegain of the amplifier 1504 should be flexible in order to meet differenttuner and application performance requirements.

As described above, the active power detector circuits 1500 and 1501 areused to monitor the power level at an input 1530 of the amplificationmodule 1402 and at its outputs 1524, respectively. By providing at leasttwo power detectors 1500 and 1501, the non-linearities of a single powerdetector can be removed from the system. The removal of thenon-linearities facilitates a more accurate gain determination from theinputs to the outputs of the module 1402.

Next, an optional differential amplifier 1526 is configured to subtractthe output power level from the input power level to produce a gaincontrol signal that is proportional to the gain of the amplificationmodule 1402 if the power detector circuits 1500 and 1501 are linear indecibels. The gain control signal can be used elsewhere in the system1400 to enable the system, for example, to comply with data over cablesystem interface specification (DOCSIS 1.1) signal level detectionrequirements. The power detector 1501 can also be used to sense theoutput power level in order to drive an AGC loop created along a path ofconnection leads 1409 and 1414, which are also shown in FIG. 14.Alternatively, outputs of the detectors 1500 and 1501 can be sensedseparately as indicated by optional respective connection leads 1540 and1545.

Moreover, the power detector 1501, in sensing the output level anddriving the AGC loop, is able to maintain the output of theamplification module 1402 at a level that maximizes the signal to noiseplus distortion ratio of all of the associated output signals. Anoptimum output level is the largest possible signal level that does notcause the downstage tuners to violate the system's distortionrequirements. In the present invention, this output signal level isprogrammable in order to provide system flexibility and compatibilitywith a wide range of associated components, such as tuners.

FIG. 16 is a schematic diagram of an exemplary circuit 1600representative of the circuit architecture of the amplifiers 1502 and1504 shown in FIG. 15. However, the size of the devices used in thecircuit 1600 can vary in order to maximize the dynamic range of thefirst stage device 1404. The circuit is a source coupled differentialtransistor pair amplifier formed by active devices 1601 and 1602 alongwith an off-chip RF choke 1614. The RF choke 1614 is used as a tail forthe common source of the devices 1601 and 1602 and is a differentialtransistor pair tail with a large AC impedance. The large AC impedancehelps second-order distortion performance and suppresses common-modeinput signals without wasting DC voltage headroom.

Next, devices 1606 a and 1608 a provide an active nonlinear load tomaximize headroom. The nonlinearity of the diode I-V curve is the sameas the differential amplifier formed by the device 1601 and 1602, thusmaking the differential output voltage very linear. If the diodes 1606 aand 1608 a are made the same size devices as in the differential pairdevices 1601 and 1602 and the current is the same, the gain would beunity. This would not result in a very useful amplifier since the noisefigures would not be too high.

If some of the DC current is shunted away from the diodes 1606 a and1608 a, the gain will be greater. Since an ideal current source hasinfinite impedance, no gain reduction results. This is shown in FIG. 16Ausing the current sources 1610 a and 1612 a.

FIG. 16B is a more detailed illustration of the circuit 1600 a shown inFIG. 16A. In FIG. 16B, N-channel metal oxide semiconductor (N-MOS)transistors 1606 b and 1608 b respectively form the diodes 1606 a and1608 a. The transistors 1610 b and 1612 b form P-channel metal oxidesemiconductors (P-MOS) current sources that are respectivelyrepresentative of the current sources 1610 a and 1612 a. Because ofconsideration for process variations, the transistors 1606 b and 1608 bwere chosen as N-MOS devices, matching the device types of thetransistors 1601 and 1602. The transistors 1606 b and 1608 b, however,could have also have been P-MOS devices.

Peaking inductors 1614 and 1616 are used to achieve better bandwidthfrom the two output ports 1503 a and 1503 b. Most on-chip inductors alsoinclude a fair amount of series resistance, which is represented by theresistors 1618 and 1620. The configuration of both diodes and resistorscreates a hybrid between a pure diode load and a pure resistive load. Apure resistive load would provide significant gain but would not be aslinear as a pure diode load. With the hybrid approach, the presentinvention provides an increase in both linearity and gain.

Additionally, active devices 1610 a and 1612 b are used to shunt apredetermined amount of current from the devices 1606 a and 1608 b inorder to reduce a voltage drop and increase the voltage headroom of thedevices 1601 and 1602, thus also increasing their linearity. While FIGS.16A and 16B provide exemplary versions of architecture used in the firststage amplifiers 1502 and 1504, FIG. 17 provides a circuit 1700representative of exemplary architecture of the second stage amplifiers1520 and 1522.

As illustrated in FIG. 17, the second stage device 1408 includes twocurrent steering circuits 1702 and 1704, which are summed together in awired OR implementation. The function of the two current steeringcircuits 1702 and 1704 is substantially similar to the current steeringarrangement described in relation to FIG. 9.

One difference, however, between the arrangements shown in FIGS. 9 and17 is that the current steering circuits 1702 and 1704 are biased withcurrent sources 1706 and 1708 respectively. The input to currentsteering circuit 1702 is the signal produced at the outputs 1503A and1503B of the unattenuated amplifier 1502 shown in FIG. 15. The input tothe current steering amplifier 1704 is the signal produced at the output1505A and 1505B of the attenuated amplifier 1504 shown in FIG. 15.

The on-chip load of output ports −Vout and +Vout are respective seriesresistor inductor shunt peaking combination 1710/1712 and 1714/1716 thatare used to extend the broadband bandwidth. While the inductors 1614 and1616 of FIG. 16 are used in a series peaking arrangement, it is pointedout that in the exemplary embodiment of FIG. 17 the series resistorinductor 1710/1712 and 1714/1716 are placed in parallel with theparasitic capacitance. Additionally, inductors 1718 and 1719 areoff-chip RF chokes used to reduce the DC voltage drop across the shuntpeaking combinations 1710/1712 and 1714/1716, respectively.

A key advantage to the autonomous AGC control technique of the presentinvention is that autonomous AGC sets an optimum level of input for anamplification stage for best noise figure and distortion performance.Further, autonomous AGC, as implemented in the present invention,provides AGC for all channels of the entire spectrum of the cable modemsimultaneously. If the current source of a standard coupled pair is usedto vary the gain, the linearity degrades substantially as the gain isreduced. This might be, for example, because the gain is proportional tothe transconductance while the linearity is proportional to the DC biascurrent. The transconductance varies with the square root of the biascurrent in a metal oxide semiconductor device. As the current isdecreased, the linearity reduces substantially faster than the gain sothat a highly distorted signal can be passed with a significant amountof power gain. However, differential pairs can be implemented in twostacked devices and consume minimal voltage headroom and have a largelinear range when the current is not varied.

Alternatively, current steering circuits vary gain by switching thecurrent from the load to the voltage supply. In this case, the linearityis always constant for the input device but the gain can now be varied.The problem is that a first stage current steering circuit would stackat least three device and use more voltage headroom than a differentialpair and will have an inferior second order linearity performance.

The architecture of the present invention blends the advantages of thehigh second order linearity differential pair in the first stage and theconstant linearity with varying gain ability of the current steeringcircuit in the second stage.

CONCLUSION

The present invention provides an active splitter arrangement in theplace of traditional splitter arrangements that can be used in devicessuch as cable set-top boxes, cable televisions, and/or cable modemapplications. By using the active splitter arrangement of the presentinvention, the full use of the input signal ranges can be achieved andthe associated amplification stages can provide better linearity andgain performance characteristics.

The present invention also provides the active splitter arrangement andits associated LNA function on a single IC to provide low noiseamplification and automatic gain control with minimal systemdegradation. Alternatively, this also includes discrete implementationsof the active splitter function, i.e., the non IC active splitters.Additionally, the present invention can be implemented using a varietyof substrate technologies, to include CMOS. The LNA function of eachdownstage tuner is incorporated in a single low noise broadband VGA thatprovides nearly identical performance to single tuners at a lower coststhan conventional passive splitter solutions.

Finally, the present invention also provides an improved circuitarchitecture that can vary the gain of low noise VGAs without degradingtheir linearity performance. The improved architecture blends theadvantages of the fixed gain high second order linearity differentialpair in a first stage and the highly linear varying gain ability of acurrent steering circuit in a second stage.

The foregoing description of the preferred embodiments provide anillustration and description, but is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Modifications andvariations are possible consistent with the above teachings or may beacquired from practice of the invention. Thus, it is noted that thescope of the invention is defined by the claims and their equivalents.

1. A circuit comprising: a differential signal splitting mechanismconfigured to (i) receive a first differential signal as an input, (ii)amplify the received first differential signal, (iii) produce from theamplified first differential signal a number of second differentialsignals, each being substantially identical to the amplified firstdifferential signal, and (iv) output each of the amplified seconddifferential signals; and a number of tuners corresponding to the numberof second differential signals, each tuner being configured to receiveone of the second differential signals output from the differentialsignal splitting mechanism and derive one or more informationaldifferential signals therefrom.
 2. The circuit of claim 1, wherein eachof the number of tuners excludes a low noise amplification function. 3.The circuit of claim 1, further comprising a variable gain amplifiercoupled to an input of the signal splitting mechanism, the variable gainamplifier being configured to receive the first differential signal asan input thereto.
 4. The circuit of claim 1, further comprisingdifferential transmission lines to couple the signal splitting mechanismto the number of tuners.
 5. The circuit of claim 1, wherein the signalsplitting mechanism is an active signal splitting mechanism.
 6. Thecircuit of claim 1, wherein the circuit is a cable set-top boxconfigured for use in at least one of a television, a cable modem, apersonal video recorder, and an out of band device.
 7. A method ofamplifying a differential signal in a device including a firstamplification module, a number of second amplification modules, and again control device, the method comprising: receiving a transmitteddifferential signal in the first amplification module and applying afirst level of amplification thereto; and providing the first levelamplified signal to each of the number of second amplification modulesto produce a corresponding number of second level amplified differentialsignals, each second level amplified differential signal havingcharacteristics substantially similar to the other of the number ofsecond level amplified differential signals.
 8. The method of claim 7,further comprising providing the number of second level amplifiedsignals to a corresponding number of tuners.
 9. The method of claim 7,further comprising: detecting a signal level of each of the number ofsecond level amplified signals; producing a gain control signal basedupon the detected signal level; and providing the gain control signal asan input to each of the amplifiers of the second amplification moduleand controlling a gain thereof based upon the provided gain controlsignal.